Device for transforming direct current into alternating current



Nov. 22, 1938. J. w. ALEXANDER DEVICE FOR TRANSFORMING- DIRECT CURRENT INTO ALTERNA TING CURRENT Filed Aug. 22, 1956 MlVE/WOR JOI/AN W ALEXANDER B) ATTORNEY Patented Nov. 22, 1938 UNITED STATES PAT'ENT OFFICE DEVICE FOR TRAN SFORIWING DIRECT CUR- RENT INTO ALTERNATING CURRENT Application August 22, 1936, Serial No. 97,306 In Germany October 15, 1935 1 Claim.

The invention relates to a device for converting direct into alternating current with the aid of a vibrating member which alternatelybreaks and closes a circuit with the result that an intermittent direct current is produced which may be converted, for example by means of a transformer, into a pure alternating current. There are preferably two circuits which are alternately broken and closed again by the member vibrating between two contacts while said two contacts are preferably connected to the ends of a primary transformer winding. One of the terminals of the source of direct current is connected to the vibrating member, the other terminal being connected to the mid-point ofthe transformer winding. The construction of such devices for a low unidirectional voltage ranging, for example, from 6 to 12 volts alfords few inconveniences. In the conversion of currents of higher voltage, for example of volts and upwards, there arise, however, drawbacks which substantially consist in that a strong sparking is set up at the points of contact, which results in a rapid wear of the contacts. It has previously been proposed to bridge the points of contact by means of condensers, which has, however, proved to be ineificient in the case of devices with a high supply voltage.

The invention has for its object to provide a device for converting direct current of comparatively high voltage (from 100 to 250 volts) into alternating current, wherein the abovementioned drawback is avoided and which, moreover, does not give rise to radio-disturbances.

According to the invention, both points of contact are bridged by the series-connection of a capacitance and a fixed resistance.

The invention will be more clearly understood byreferring to the accompanying drawing which represents, by way of example, one embodiment thereof.

A vibrating member I is furnished at its free end with two contacts 2 and 3 and is maintained in vibratory motion between contacts 4 and 5 which preferably are arranged so as to be slightly elastic. The member I is connected to one of the terminals of a source of direct current which is connected to terminals 6 and theother terminal of which is connected to the mid-point of a primary transformer winding l3. The contacts 4 and 5 are connected to the ends of said transformer winding. The transformer is constructed, for example, as an auto-transformer. To the terminals 1 may be connected a wireless receiving set adapted to be supplied by alternating current. The vibrating member is kept in vibratory movement by means of a coil 8 connected between the fixed point of said member and one of the contacts, for example, the contact 4. In order to adapt the device to different supply voltages,

V the coil has connected in series with it a resistance The current flowing through the contacts 2, 4

and 3, 5 respectively is periodically switched oh? and on, which results in that each hali' of the transformer winding is traversed by an intermittent direct current. As is well-known, a spark is produced at the interruption, more particularly if, as in the present case, the circuit contains a high self induction and it has previously been proposed to prevent sparking by shunting the contacts by means of condensers. Thus it is possible to obtain a device which operates in a quite satisfactory manner in the case of low supply voltages. It has been found, however, that with a high direct voltage the production of sparks cannot be suppressed in this way, which is due to the following:

Assuming that one of the contacts is closed, it will be seen from the wiring diagram thatat the condenser connected in parallel with the other contact almost the full transformer voltage, that is to say, double the direct voltage, is operative, so that this condenser is charged up to said voltage. A moment afterwards the contact is opened again while the other contact is closed with the result that the condenser connected in parallel with the latter contact partly discharges through thesecontacts. I

It has been found, that the drawbacks arise from said discharge current as the latter causes a strong sparking. According to the invention, there is connected in series with the condensers l0 and ll a resistance l2 which should preferably be common to said two condensers and which is connected in this case between the Vibrating member and the common point of two condensers connected in series between the contacts l and 5. This resistance limits the discharge current and consequently prevents the production offa spark at make. The exact value of said resistance is connected with the capacity of the condensers: if it is chosen too high,.the spark at break becomes too intense. It has, however,

proved to be possible to give the resistance a value such that at the contacts substantially no sparks are produced or at least the production of sparks is no longer harmful. Besides, the resistance l2 prevents the production of high-frequency oscillations which might have an injurious eifect on the radio-reception.

The condensers l0 and H constitute an additional load on the transformer winding l3, said load being not at all negligible in dimensioning the transformer in the case of the high voltages occurring in the case under consideration. According to the invention these condensers are therefore given a slight capacity, that is to say a capacity lower than 0.25 i ,f, a favorable value being 0.1 1. The additional load is in this case small in comparison, for example, with the useful load occurring with the supply of wireless receiving apparatus. It has been found that favorable results may be obtained in this case with a resistance l2 having a value of about 60 ohms.

What I claim is: In a device for converting direct current into alternating current by means of a vibratory system having a vibrating member adapted upon connection of the system to a source of direct current to vibrate between two contacts to thereby alternately and periodically make and break "the circuits which include a transformer winding and the source of direct current whereby current is made to flow first in one of the circuits and then in the other thereof, in combination, a circuit comprising a pair of condensers in series connected between said two contacts and means for limiting the discharge current of the said condensers comprising a resistance element of the order of 60 ohms connected directly between the vibrating member and a point of the last named circuit which is common to the two condensers.

J OHAN WILLEM ALEXANDER. 

